Breen later recalled: "...we took the action deliberately, having thought over the matter and talked it over between us. Treacy had stated to me that the only way of starting a war was to kill someone, and we wanted to start a war, so we intended to kill some of the police whom we looked upon as the foremost and most important branch of the enemy forces ... The only regret that we had following the ambush was that there were only two policemen in it, instead of the six we had expected...

During the Irish War of Independence, Breen had a £10,000 price on his head, however, he quickly established himself as a leader within the Irish Republican Army (IRA). Numerous stories are known about his heroism, one of which details the rescuing of his comrade Seán Hogan at gunpoint from a heavily guarded train at Knocklong station in County Limerick. Another incident occurred in Dublin when he shot his way out through a British military cordon in the northern suburb of Drumcondra (Fernside) in which he and volunteer Sean Treacy escaped only for Treacy to be killed soon after. Breen was shot at least four times, once being in the lung for the second time (the first being in the Knocklong rescue). He was present at the ambush in Ashtown on the Meath/Dublin border where Martin Savage was killed while trying to assassinate the Lord Lieutenant of Ireland, Sir John French, 1st Earl of Ypres.

In the June 1922 elections Breen was selected as a candidate for Waterford and Tipperary East by the pro- and anti-Treaty sides and was returned unopposed.

Irish Civil War

Breen was elected to Dáil Éireann in 1923 as a Republican, anti-Treaty Teachta Dála (TD). Following the Anglo-Irish Treaty, Breen joined the Anti-Treaty IRA in the unsuccessful civil war against his former comrades. He was arrested by soldiers of the Irish Free State and interned at Limerick Prison. He spent two months here before going on Hunger strike for 6 days followed by going on thirst strike for six days. Dan Breen was then released.

Death

He died in Dublin in 1969 and was buried in Donohill, near the place of his birth. His funeral was the largest seen in West Tipperary since his close friend and comrade-in-arms, Seán Treacy was buried at Kilfeacle in October, 1920. An estimated attendance of 10,000 mourners assembled in the tiny hamlet, giving ample testimony to the esteem in which he was held. Breen was the subject of a 2006 biography, Dan Breen and the IRA (Mercier Press), by Joe Ambrose.